Atmospheric Drag

About the Image:

The atmosphere expands dramatically at the high altitude during space weather disturbances. Within the red regions in the image on the left, the atmospheric density has increased by values in excess of 20% during a relatively mild magnetic storm. During major storm events, these values can easily exceed 100%. More importantly these are unpredicted and irregular changes in the atmospheric density experienced by satellites in low-Earth orbit. Following a space weather disturbance, literally thousands of objects in space alter their predicted orbits and must be relocated by satellite tracking stations. (Courtesy of A. Burns, University of Michigan)

A satellite, orbiting around the Earth, would continue to orbit forever if gravity were the only force acting on it. However, satellites below 2000 kilometers, are actually travelling through the Earth's atmosphere. Collisions with air particles, even at these high altitudes, slowly act to circularize the orbit and slow down the spacecraft causing it to drop to lower altitudes.

How is Space Weather Involved?

The density of the air particles responds sensitively to the solar activity, season, longitude, latitude, local time and magnetic storm conditions. Emissions from the Sun (including, the highly variable xray and ultraviolet output) cause the upper atmosphere to heat and expand. These energetic solar outputs increase dramatically during periods of high solar activity. If magnetic activity is also triggered at Earth, intense currents flowing through the upper atmosphere, and energy deposited by high speed particles at high latitudes dramatically increases the heating and expansion of the atmosphere in these regions.

How Long Do Satellites Live?

The lifetime of a satellite is strongly dependent on its altitude. At 300 km altitude it may last for 20-50 days (depending on the Sun's activity level) before it reenters and burns up. However, at 180 km, this lifetime reduces to mere hours.

Satellite Reentry - The Skylab Problem

With all of the uncertainties in the prediction of atmospheric density along the satellites orbital path, even our most sophisticated programs are unable to determine the time of a satellite's reentry to better than 10% accuracy or within 2.4 hours in a 24 hour period. Satellites are moving so fast that within 2 hours it has completely circled the globe. The bottom line is that it is not possible to predict the location of the reentry. A better knowledge of space weather and climate and the response of Earth's upper atmosphere is required to make progress on this issue.
The reentry location is rarely a problem because most of the satellite is totally vaporized during the decent. However, occasionally, such as in the case of Skylab which was an extremely large piece of space hardware, parts of the spacecraft may reach the ground.